Method for forming a nanoporous grain boundary structure

ABSTRACT

Gadolinium-doped cerium oxide slurries used to form a patchwork type surface structure with nanoporous grain boundary prepared by mixing gadolinium-doped cerium oxide and a polymer binder to form a first mixture; wet-atomizing the first mixture under a pressure of at least 100 MPa to obtain a second mixture; coating the second mixture to a substrate to form in a coated substrate; and sintering the coated substrate. The patchwork type structure is a polygonal or honeycomb structure having a size of from 0.1 μm to 3 μm.

BACKGROUND

Field of the Disclosure

The present disclosure relates generally to a method for preparing ametal oxide slurry, its use for forming an electrolyte layer having apatchwork-type surface structure with a nanoporous grain boundary, andthe electrolyte layer thereby formed. The metal oxide can begadolinium-doped cerium oxide, zinc oxide, or anything else. The metaloxide slurry prepared by the method is useful in fabricating a thin anddense electrolyte layer for fuel cells, as well as in manufacturing thinfilm solar cells and as a transparent conductive oxide for solar energystorage.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentinvention.

As a renewable and sustainable energy source alternative to fossil fuel,solar energy has been attracting a lot of attention. Varioustechnologies, including solar heating, solar photovoltaics, and solarthermal energy technologies have been utilized to capture solar energy.Although solar energy is the most abundant renewable energy,conventional solar energy devices utilize only a fraction of availablesolar. There is an urgent need to develop a device having highutilization efficiency of solar energy. One method to effectivelycapture solar energy is in the form of chemical bonds, as inphotosynthesis, brought about by splitting water photoelectrochemically.

Development of semiconductors, semiconductor materials and ceramicmaterials capable of directly converting sunlight into fuels may providea singular solution to convert, capture and store solar energy.Homeowners could use solar panels during the day to generate power totheir home by using solar-generated stored energy to split water intohydrogen and oxygen for later combustion and further energy generation.At night, the stored hydrogen and oxygen can be recombined, using a fuelcell to generate electrical and thermal power while the solar panelswould otherwise be inactive.

Transparent conductive oxide (TCO) is an optically transparent andelectrically conductive doped metal oxide used for thin film siliconsolar cells. TCO has to transmit as much light as possible through asubstrate window to the active light-absorbing material beneath, as wellas carry the current as an ohmic contact with minimal resistive losses.Indium tin oxide, fluorine-doped tin oxide, and doped zinc oxide aregenerally used as an inorganic film of a layer of TCO. Among them, dopedzinc oxide is a strong TCO candidate to store solar energy due to itshigh transparency and high conductivity. Various attempts have been madeto deposit a ZnO based TCO thin film to lower the resistivity.

Fabrication of thin films used as electrolytes for solid oxide fuelcells (SOFCs) has been intensively studied recently in an effort toreduce the operating temperature of SOFCs. SOFCs convert chemical energyof a fuel directly to electrical energy. Due to its high energyconversion efficiency and fuel flexibility, SOFCs have a wide variety ofapplications. However, SOFCs operate at a high temperature—from 500 to1000° C.—and utilize a solid oxide or a ceramic as an electrolyte. Suchhigh operating temperature limits commercial use of this technologybecause acceptable performance could only be achieved from a very smallnumber of cells, and configuration of a commercial scale power systemwith such a small number of cells is not cost-effective.

Ceria-based electrolytes are of current interest for application inintermediate temperature-solid oxide fuel cells (ITSOFCs) due to theirhigh ionic conductivity. Gadolinium doped ceria (GDC), for example, hassignificantly higher ionic conductivity than that of yttria stabilizedzirconia. Suzuki et al. have reported that microtubular cells cangenerate over 1 W cm⁻² at 550° C. with a ceria-based electrolyte[Suzuki, T., Zahir, H., Funahashi, Y., Yamaguchi, T., Fujishiro, Y., andAwano, M., 2009, “Impact of anode microstructure on solid oxide fuelcells,” Science, 325, pp. 852-855—incorporated herein by reference].However, at lower temperatures, the conductivity of ceria-basedelectrolytes significantly decreases. Ohmic losses from the electrolytecan be minimized through the use of a thinner electrolyte [see: Leah, R.T., Brandon, N. P., and Aguiar, P., 2005, “Modeling of cells, stacks andsystems based around metal-supported planar IT-SOFC cells with CGOelectrolytes operating at 500-600 ° C.,” J. Power Sources, 145, pp.336-352; Suzuki, T, Zahir, H, Yamaguchi, T, Fujishiro, Y, Awano, M.Fabrication of micro-tubular solid oxide fuel cells with asingle-grain-thick yttria stabilized zirconia electrolyte. J. PowerSources 2010; 195:7825-7828; and Suzuki, T., Zahir, H., Funahashi, Y.,Yamaguchi, T., Fujishiro, Y., and Awano, M., 2008, “Fabrication andCharacterization of Microtubular SOFCs with Multilayered Electrolyte,”Electro. & Solid-State Letters, 11(6), pp. B87-90; each incorporatedherein by reference]. Highly dispersed nano-size GDC slurry withhomogeneous distribution is indispensable for fabricating a dense andthin electrolyte layer.

Recently, in the chemical engineering and food technology fields, a wetatomizing system has been developed as a new method of mixing anddispersing [Zahir, Md. H., Suzuki, T Yamaguchi, I., Fujishiro, Y., andAwano, M., 2009 “Wet atomization of Gd-doped CeO₂ electrolyte slurriesfor intermediate temperature microtubular SOFC application” Fuel Cells,9, pp. 164-169 incorporated herein by reference]. Using this system,particle size reduction and homogenization are achieved within a shortperiod of time. Therefore, attempts were made to synthesize the GDCslurries through the use of wet atomizing systems for the preparation ofnanosized particles. The wet atomizing system divides the pressurizedfluid in one channel and creates a cross-collision for atomization,emulsification, and dispersion. Dispersion by means of a high-pressureatomizer is performed by the large shearing force generated when aliquid is passed through an extremely narrow (small) gap at high speed.As a result, a fine homogeneous solid solution could be obtained withina very short time. It has been reported that the pore-size distributionsof gamma-Al₂O₃ membranes with the addition of a 3.5 wt. % solution ofpolyvinyl alcohol (PVA) polymer do not show a measurably altered porestructure [Schoonman, J., 2003, “Nanoionics”, Solid State Ionics, 157,pp. 319-32—incorporated herein by reference]. Therefore, theoptimization of the amount of binder polymer for fabrication of a smoothcrack-free electrolyte layer (membrane) is important.

Zahir et al. previously reported a homogeneous GDC electrolyte slurryprocessing system through the use of wet-atomization; the fabricated.SOFC with the atomized electrolytes showed a maximum power density ofonly 350 mW cm⁻² at 500° C. [Zahir, Md. H., Suzuki, T., Yamaguchi, T.,Fujishiro, Y., and Awano, M 2009 “Wet atomization of Gd-doped CeO₂electrolyte slurries for intermediate temperature microtubular SOFCapplication” Fuel Cells, 9, pp. 164-169—incorporated herein byreference]. However, the effect of the binder content has not yet beenreported.

The present disclosure describes a method and electrolyte layer thatsolves the above problems. The present disclosure describes a method inwhich a higher amount of binder can be used to fabricate a nanoporousnatural patchwork-type surface structure which has potentialapplications in other fields.

SUMMARY

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

It is an object of the present disclosure is to describe the effect ofthe binder (polymer) content in an electrolyte slurry.

It is a further embodiment of the disclosure to provide a method forusing a metal-oxide slurry in a wet-atomization process.

It is a further embodiment of the disclosure to provide awet-atomization process using particular characteristics of atomizerpressure and number of cycles for the preparation of the electrolyteslurry.

It is a further object of the disclosure to provide a process for thesynthesis and fabrication of a GDC thin layer.

It is a further embodiment of the disclosure to provide an electrolytefor SOFC application.

It is a further embodiment of the disclosure to provide a method toprepare a metal oxide layer having with a nanoporous grain boundary.

It is a further embodiment of the disclosure to provide an electrolytelayer having a nanoporous grain boundary and its use to fabricate anSOFC and/or in solar energy storage technologies.

It is a further embodiment of the disclosure to provide a method forpreparing metal oxide slurry and to form a patchwork surface structurewith a nanoporous grain boundary using high pressure wet-atomization.

In a further embodiment zinc oxide is used as the metal oxide, which isuseful for solar energy storage.

In another embodiment a gadolinium doped ceria (GDC) is used as themetal oxide, and the slurry contains polyvinyl butyral as a binder.

In another embodiment it is an object to provide a method that, afterthe wet-atomization, forms a substrate coated with the GDC slurrieswhich is subsequently co-sintered at 1400° C. to obtain a GDCelectrolyte exhibiting a unique patchwork-type surface structure havinga nanoporous grain boundary.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1A is a microstructure (×10,000) of the electrolyte surfacesintered at 1250° C. for 1 hour after coating with atomized GDC mixedwith 8 wt. % polyvinyl butyral (PVB) polymer on top of the anodesupport.

FIG. 1B is a microstructure (×10,000) of the electrolyte surfacesintered at 1400° C. for 1 hour after coating with atomized GDC mixedwith 8 wt. % PVB polymer on top of the anode support.

FIG. 1C is a microstructure (×10,000) of the electrolyte surfacesintered at 1400° C. for 1 hour after coating with atomized GDC mixedwith 16 wt. % PVB polymer on top of the anode support.

FIG. 1D is a microstructure of the GDC electrolyte shown in FIG. 1C athigher magnification (×20,000).

FIG. 1E is a microstructure of the GDC electrolyte shown in FIG. 1C athigher microstructure (×30,000).

FIG. 2A shows electrolyte surface image (×30,000) sintered at 1250° C.for 1 hour after coating with 16 wt. % PVB polymer.

FIG. 2B is an enlarged image (×120,000) of the electrolyte surface imageof FIG. 2A.

FIG. 3A is a microstructure (×5,000) of the electrolyte surface sinteredat 1400° C. for 1 hour after coating with atomized GDC mixed with 16 wt.% pyrrolidinone polymer on top of the anode support.

FIG. 3B is a microstructure (×5,000) of the electrolyte surface sinteredat 1400° C. for 1 hour after coating with atomized GDC mixed with 16 wt.% polytetrafluoroethylene polymer on top of the anode support.

FIG. 4A shows the GDC-NiO tube coating with GDC electrolyte sintered at1400° C.

FIG. 4B shows the extruded GDC-NiO tube.

FIG. 4C shows the cross-sectional image of the GDC-NiO tube of FIG. 4A.

FIG. 4D shows the porous anode microstructure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views.

A gadolinium-doped cerium oxide slurry having a patchwork-type surfacestructure having a nanoporous grain boundary was prepared by mixinggadolinium-doped cerium oxide and a polymer binder to form a firstmixture, wet-atomizing the first mixture under high pressure to obtain asecond mixture, coating the second mixture to a substrate, and sinteringthe coated substrate to form a layer having a patchwork-type structurein a polygonal or honeycomb structure form. The patchwork-type structurehas polygonal grains having a size of from 0.1 μ.m to 3 μm, preferably0.5-2 μm or 1.0-2.0 μm, and a nano-porous grain boundary havingnanopores with a diameter of from 0.01 μm to 0.7 μm.

The pressure used during wet atomization while preparinggadolinium-doped cerium oxide slurries is from 100 to 200 MPa,preferably 130 to 170 MPa, more preferably approximately 150 MPa.Particular fluid flow pressure provides ultrafine GDC slurries withhighly-dispersed and homogeneous distribution.

EXAMPLES

A fluid mixture and/or suspension containing a Gd oxide was pumped underdifferent pressure during atomization. When the fluid mixture and/orsuspension was atomized by applying a pressure of 100 MPa to the samesample 3 times, the mean particle size diameter was about 0.67 μm. Theparticle size was shifted to a higher value after atomization by apressure of 200 MPa. This might be due to the agglomeration of theparticles. Actually, the higher the pressure, the greater thetemperature rise in the liquid. This high pressure promotes thedeterioration of the dispersion liquid component and agglomeration. Itwas determined that a high pressure of 150 MPa or less, preferably 100MPa or less is required in order to achieve ultrafine GDC slurries withhigh dispersion.

The wet-atomizing process was repeated up to 5 times, preferablyrepeated 3 times. Next, the number of atomization process cycles wasdetermined while keeping the same parameters over the same sample(repeated 1-5 times). The mean particle size's diameter (D₅₀) was 0.36μm after atomizing only one time at a pressure of 150 MPa. The particlesize distribution was shifted to a lower value after repeating theprocess three times at a pressure of 150 MPa. The particle sizedistribution was shifted slightly to higher values after repeating theatomizing five times. The particle sizes were in the range of 100-200 nmand homogeneously dispersed. The particles were spherical, and formedpatchwork-type or honeycomb structure layers in various sizes.

The fluid mixture used in preparing gadolinium-doped cerium oxideadditionally contains ethanol and an organic solvent such as toluene orsuspension medium. The fluid mixture preferably contains ethanol andtoluene as a solvent, preferably in combination with water The fluidmixture used in preparing gadolinium-doped cerium oxide preferablycontains an amine dispersant

To form a gadolinium-doped cerium oxide electrolyte layer the slurry issintered on a substrate using a sintering temperature of from 1300 to1450° C., preferably from 1350 to 1425° C., or about 1400° C.

Very smooth surfaces were observed by using atomized GDC slurries uponan addition of 8 wt. % PVB polymer (FIGS. 1A and 1B). FIGS. 1A and 1Bshow the SEM images of the surface of the thin slurry layer co-sinteredat 1250° C. and 1400° C., respectively. When the substrate coated withthe GDC slurries prepared by the wet-atomization method was sintered at1400° C., the grain size became larger, ranging approximately from I pmto 6 μm, compared to that of the coated substrate sintered at 1250° C.

FIG. 1B shows a crack-free, smooth and dense GDC electrolyte having apatchwork-type surface structure, but no nanoporous grain boundary wasobserved. It is observed that grain growth and densification took placewithout creating any cracks on the electrolyte surface. Below 1300° C.,some hole-like structure are observed at some of the grain boundaries(see FIG. 1A), which disappear at above 1400° C. (see FIG. 1B). The SEMimage (see FIG. 1A) of a GDC atomized sample that was sintered at 1250°C. for 1 h showed that all of the grains were fairly regular. Most ofthe grains were smaller than 0.5 μm (500 nm) in size and some of thegrains were as small as 0.2 μm (200 nm). This is an important finding,since the presence of ultrafine grains is known to enhance manyproperties, such as ionic conductivity and mechanical strength[Schoonman, J., 2003, “Nanoionics”, Solid State Ionics, 157, pp.319-32—incorporated herein by reference].

The fluid first mixture used in preparing gadolinium-doped cerium oxideslurries preferably contains polyvinyl butyral (PVB) as a binder. Theamount of the PVB in the first mixture is preferably from 12 to 20%,more preferably from 14 to 18%, or about 16% by weight based on thetotal weight of the fluid mixture or suspension. A nanoporous grainboundary was found to form when the fluid mixture used to form anelectrolyte layer contained 16 wt. % PVB polymer. It is remarkable tonote that the nanoporous grain morphology was formed on all of thesurroundings of all of the grains in respect to smaller and larger sizes(see FIGS. 1C, 1D and 1E). FIGS. 2A and 2B show microstructures of theGDC slurry surface sintered at 1250° C. at magnifications of ×30,000(FIG. 2A) and ×120,000 (FIG. 29). The hole-like structures, ranging from0.1 μm to 0.3 μm, including holes having a diameter of 0.2 μm, as wellas tiny pores at some of the grain boundaries of the same range ofdiameter, are observed even at 1250° C.

The polymer binder used in preparing gadolinium-doped cerium oxide ispreferably neither polyvinyl pyrrolidinone (PVP) norpolytetrafluoroethylene (PTFE), and/or contains no amounts of thesepolymers. The effect of different polymers, such as PVP and PTFE, intothe electrolyte slurry was tested. FIG. 3A shows a microstructure(×5,000) of the electrolyte surface sintered at 1400° C. for 1 hourafter coating with atomized GDC mixed with PVP polymer (16 wt. %) on topof the anode support. FIG. 39 shows a microstructure (×5,000) of theelectrolyte surface sintered at 1400° C. for 1 hour after coating withatomized GDC mixed with PTFE polymer (16 wt. %) on top of the anodesupport. When PVP or PTFE polymer is used instead of PVB as the binder,the surface of the GDC slurry became less smooth and contains a numberof hole-like structures. However, no nanoporous structure in the grainboundary was observed (FIGS. 3A and 3B), even after sintering at 1400°C. This may be due to the low solubility of PVP and PTFE into a tolueneand ethanol mixture. It looks like natural patchwork-type morphology.

At least three essential parameters seem to be important in order tofabricate a nanoporous patchwork type morphology; namely: (i) the use ofa fluid mixture or slurry containing an excess (e.g., 16 wt. %) of PVBpolymer; (ii) co-sintering at high temperature, e.g., above 1400° C.;and (iii) sintering on a highly porous preferably ceramic support. Theporous netting morphology was fanned only in the periphery of theboundaries of all grains, as shown in FIGS. 1C and 1D with amagnification 20,000 and 30,000, respectively. FIGS. 1C, 1D and 1E showmicrostructures of the GDC slurry surface sintered at 1400° C. The samearea was magnified at ×10,000 (FIG. 1C); ×20,000 (FIG. 1D); and ×30,000(FIG. 1E). FIG. 1C shows a reduction of the constituent particles whenthe amount of PVB is increased from 8% to 16%. FIG. 1E shows a very fineporous structure at all of the grain boundaries, which is suitable forpreparing a dense electrolyte thin film. The patchwork-type morphologymight be due to abnormal interfacial energy and the homogeneousdistribution of fine polymer particles. It may be possible to separate asingle grain after the fabrication of the unique nonporous morphology.This might be possible because the nonporous grain boundary was formedon top of the highly porous GDC-NiO support.

FIG. 4 shows an extruded GDC-NiO tube and its cross-sectional imagecoated with GDC. FIG. 4 also shows porous anode microstructure and aninside image (FIG. 4A-4D). [FIG. 4B shows as extruding GDC-NiO tube andFIG. 4A shows after coating GDC electrolyte sintered at 1400° C.

The above nanoporous grain boundary was observed when the slurry of GDCwas coated on the top of as-extruded anode tubes (support) and thedip-coated anode tubes were co-sintered at 1400° C. for 1 h in air. Aporous anode support was obtained after co-sintering at 1400° C. for 1 hin air. The electrochemical performance of a Solid. Oxide Fuel Cell(SOFC) was extensively improved when the size of constituent particleswas reduced so as to yield a highly porous anode (support)microstructure.

This patchwork-type nanoporous with netting boundary fabricationtechnology is a contrary phenomenon in comparison with the Zener pinningeffect, because the Zener pinning effect did not consider the influenceof porous support and high temperature co-sintering [Flewitt, P. E. J.,Wild, R. K., 2001, “Grain Boundaries: Their Microstructure andChemistry,” Wiley publication; 1 edition, ISBN-10:0471979511—incorporated herein by reference]. Studies of a porousceramics grain boundary are hardly found in the available literatures.An understanding of porous boundary formation and its structure willbecome a crucial step in helping to draw new processing strategies [see:Lin, C. J., and Wei, W. C. J., “Grain boundary pinning ofpolycrystalline Al₂O₃ by Mo inclusions,” 2008, Mater. Chem. Phy., 111,pp. 82-86; Kageyama, Y., Murase, Y., Tsuchiya, T., Funabashi, H., andSakata, J., 2002, “ Formation of porous grain boundaries inpolycrystalline silicon thin films,” J. Appl. Phys., 91, pp. 9408-9413;and Suzuki, T., Zahir, H., Funahashi, Y., Yamaguchi, T., Fujishiro, Y.,and Awano, M., 2008, “Fabrication and Characterization of MicrotubularSOFCs with Multilayered Electrolyte,” Electrochem. Solid State Lett.,11, pp. B87-90;—each incorporated herein by reference]. It is anextraordinarily surprising finding because no research has yet beenreported which shows a nanoporous patchwork-type grains morphology. Thistechnology might play an important role in materials processing, grainrecovery and separation. Therefore, the present technology is ofinterest to scientists in other fields.

A metal oxide film having a patchwork-type surface structure with ananoporous grain boundary was also prepared by wet-atomizing metal oxideunder a pressure of at least 100 MPa to obtain a fluid mixture orsuspension, and sintering the fluid mixture or suspension to form themetal oxide film. The metal oxide film may be a gadolinium-doped ceriumoxide film or zinc oxide film. An integrated photoelectrochemical devicecan be constructed using this technology.

The present invention was achieved by an atomization technique and aunique natural patchwork-type nanoporous grain boundary was obtainedwhen the polymer content was 16 wt. % or greater in the slurries. Theresults of this study show that polymers (binder) can be used not onlyto fabricate a dense electrolyte, but also to generate a nanoporousgrain boundary. This technology can be applied in the field of solarenergy storage.

Method and Materials GDC Slurry Preparation

The GDC slurry was prepared by mixing commercially availableGd₀₋₂Ce₀₋₈O_(2-x) (Shin-Etsu Chemical Co., Ltd., Japan, 99.99%) powderwith a specific surface area of about 12.6 m²g⁻¹, ethanol and toluene asa solvent, polyvinyl butyral as a binder and an amine as a dispersant.The slurries were then stirred for 2 h before atomization. In thepresent invention, the final mixing process was carried out by (a)atomization and also by (b) ball milling for the purpose of comparison.The ball milling process was performed for 24 h using high grade ZrO₂balls with a diameter of 3 mm.

Wet Atomizing System

The atomization of GDC slurry was carried out by using a wet atomizer. Around shaped steel device with an extremely narrow hole was used in thepresent by first washing in distilled water followed by ultrasoniccleaning for 30 min in ethanol.

Dispersion by means of a high pressure is performed by passing a metaloxide-containing liquid through an extremely narrow (small) gap at highspeed with a pressure of 100-200 MPa. By changing the fluid flowpressure namely 100, 150 and 200 MPa and repeating the atomizationprocess up to 5 times, an a highly dispersed GDC slurry is formed foruse as an electrolyte precursor. The slurries were collected into acontainer after each step of atomization. The slurries prepared in thismanner were stored in an air-tight bottle at 4° C. until ready to use.

Md. H. Zahir &. T. Suzuki, “Effects of Polymer Binder in ElectrolyteSlurries and Their Microtubular SOFC Applications,” Journal of Fuel CellScience and Technology, April 2013, Vol. 10 is incorporated herein byreference in its entirety.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in the art, the present invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting of the scopeof the invention, as well as other claims. The disclosure, including anyreadily discernible variants of the teachings herein, define in part thescope of the foregoing claim terminology such that no inventive subjectmatter is dedicated to the public.

1. A method for preparing a gadolinium-doped cerium oxide slurry havinga patchwork surface structure with a nanoporous grain boundary, themethod comprising: mixing a gadolinium-doped cerium oxide and a polymerbinder, thereby forming a first mixture having 12 to 20% by weight ofthe polymer binder; wet-atomizing the first mixture thereby obtaining asecond mixture; coating the second mixture on a substrate, therebyforming a coated substrate; and sintering the coated substrate, whereinthe patchwork structure has polygonal grains having a size of from 0.1μm to 3 μm; and wherein the nanoporous grain boundary is present on thesurroundings of the polygonal grains.
 2. The method of claim 1, whereinthe pressure during the wet atomizing is about 150 MPa.
 3. The method ofclaim 1, wherein the pressure during the wet atomizing is from 130 to170 MPa.
 4. The method of claim 1, wherein the wet-atomizing is repeatedup to 5 times.
 5. The method of claim 1, wherein the wet-atomizing isrepeated 3 times.
 6. (canceled)
 7. The method of claim 1, wherein thepolymer binder is polyvinyl butyral.
 8. (canceled)
 9. The method ofclaim 1, wherein the polymer binder is not polyvinyl pyrrolidinone. 10.The method of claim 1, wherein the polymer binder is notpolytetrafluoroethylene.
 11. The method of claim 1, wherein the firstmixture further comprises ethanol and toluene as a solvent or suspensionmedium.
 12. The method of claim 1, wherein the first mixture furthercomprises an amine as a dispersant.
 13. A gadolinium doped cerium oxideelectrolyte prepared by the method of claim
 1. 14. (canceled) 15.(canceled)
 16. The method of claim 1, wherein the nanoporous grainboundary has nanopores with a diameter of from 0.01 μm to 0.7 μm. 17.The method of claim 1, wherein the sintering is sintering at atemperature of from 1350 to 1425° C.
 18. The method of claim 1, whereinthe first mixture has 14 to 18% by weight of the polymer binder.
 19. Themethod of claim 1, wherein the first mixture has about 16% by weight ofthe polymer binder.
 20. The method of claim 1, wherein the polygonalgrains have a size of from 0.5 μm to 2 μm.
 21. The method of claim 1,wherein the coated substrate is sintered at a temperature of 1,300 to1,450° C.
 22. The method of claim 1, wherein the nanoporous grainboundary is present on all of the surroundings of all of the polygonalgrains.
 23. The method of claim 1, wherein the first mixture iswet-atomized at a pressure of 100 to 200 MPa.